Every year in October, World Pain Day is celebrated, and it's a good opportunity for us to take a closer look at a subject that affects everyone.
In fact, two out of three French people consult their doctor regularly for pain. Pain is the subject of numerous clinical studies designed to understand the mechanisms involved and develop new treatments.
What is pain?
According to the official definition of the International Association for the Study of Pain (IASP), pain is an unpleasant sensory and emotional experience associated with, or resembling, that of actual or potential tissue damage.
Every living thing is bound to experience pain at some time or other. It is an alarm bell indicating that something in the individual's body is malfunctioning.
It is a highly subjective symptom, which can vary greatly from one individual to another, depending on the emotional, socio-cultural, ethnological or religious context in which the pain occurs.
Although pain is subjective, tools such as pain questionnaires and pain scales can be used to describe its manifestations, and to measure its intensity and impact on quality of life. In fact, all these parameters can largely modulate the perception of pain and demonstrate the existence of a close link between 'pain' and 'psychosocial context'. Brain imaging shows that the brain centres responsible for pain perception are linked to the emotional centres!
What's more, the brain and spinal cord have a powerful pain-control system that uses endorphins to regulate the transfer of painful information from the body. This system can be controlled, as in the case of top athletes who continue to play despite injury, or the fakir who is able to sleep on a nail board.
Pain: a therapeutic reference point
Despite differences in perception of the painful symptom, it is the symptom itself that enables practitioners to make a diagnosis. They find out where the pain is located (cervical, lumbar, abdominal, etc.), what it is (burning, pinching, tugging, dull pain, pressure, etc.), how it appears (at the first movements, during sport, after exercise, etc.) and whether it is accompanied by other symptoms such as fever, digestive problems, neuralgic irradiations, skin rashes, etc.
During a consultation, patients should be able to provide their therapist with these details, so that the therapist can make a rapid diagnosis and implement the most appropriate treatment.
The duration of the pain also contributes to the diagnosis and sometimes modifies the treatment.
Acute or chronic?
Acute pain acts as an alarm, allowing the body to react quickly and protect itself against mechanical, chemical or thermal stimuli. Following these intense stimuli, an information transmission mechanism is immediately triggered from the nerve endings (pain receptors called nociceptors, located in the skin, muscles, joints, viscera, etc.) to the brain.
There are several types of nociceptors, each specialising in the transmission of a particular sensation: sting, burn, temperature, pressure, etc. When activated by a danger, these nerve endings transform the information received into electrical impulses.
For example, in the case of a hand inadvertently placed on a hot plate, these electrical impulses originate in the skin nociceptors and propagate along the nerves thanks to the successive activation of ion channels present all along these fibres. These channels are also the targets of anaesthetic products.
The information thus travels via the spinal cord to the brain: only then is the signal identified and perceived as painful.
However, the hand has been reflexively released from the heating plate even before this decoding, via a reflex arc located within the spinal cord thanks to neurotransmitters such as GABA or endomorphins.
When acute pain persists for more than three months, it develops into chronic pain and loses its significance as an alarm signal: pain is no longer a symptom but becomes an illness. This category includes muscle pain, joint pain, migraines and pain associated with nerve damage. In this case, they are classified according to the pathophysiological mechanisms involved (inflammatory, neuropathic, mixed (combining inflammatory and neuropathic), or nociplastic pain (linked to alterations in pain sensors, particularly in people suffering from fibromyalgia, chronic headaches, etc.).
Major advances have been made in our understanding of pain in recent years, particularly with regard to the mechanisms involved in chronic pain.
It has also been shown that pain is not solely 'neuronal': the glial cells of the central nervous system and certain immune cells are also involved in the onset of pain, particularly neuropathic pain. If certain glial functions are impaired, these cells secrete substances (gliotransmitters) that stimulate the sensory neurons and exacerbate the pain.
How can pain be relieved?
Medication
Inflammatory pain is now well treated with the standard analgesics: paracetamol, aspirin, anti-inflammatories and morphine and its derivatives for the most intractable pain. Although effective for acute pain, these drugs have significant side-effects (gastric and renal disorders, tolerance and dependence on morphine, etc.) if they are used for prolonged or even chronic periods.
Neuropathic pain, linked to damage to the peripheral or central nervous system, responds very poorly to previous analgesics, apart from certain opioids. However, the long-term side-effects of opioids mean that they cannot be used for chronic pain. As a result, the main treatments now used to treat neuropathic pain are antidepressants and antiepileptics. While these two types of medication have fewer side-effects, they are only moderately effective, and can be observed in only around 50% of patients. Local treatments can also be used in the form of patches (local anaesthetics or capsaicin) or injections when the pain is not too widespread.
Biological, genetic and clinical response markers are being researched to enable tailor-made treatment to be provided, so as to avoid not only treating certain people unnecessarily with a drug that is ineffective in their case, but also missing out on sub-groups of patients who respond to a new therapeutic approach. These markers are derived from the results of psychophysical tests, which enable a very detailed assessment of symptoms, in order to distinguish subtypes of patients with the same type of pain. Once identified, these markers are used to determine whether a patient suffering from neuropathic pain should be treated with antidepressants or antiepileptics.
Botulinum toxin has also recently been used to combat peripheral neuropathic pain, when previous treatments have not been sufficiently effective. It is administered by subcutaneous injections and lasts for three months (sometimes longer), with no notable side-effects. However, because of the way it is administered, botulinum toxin is reserved for superficial neuropathic pain that does not affect too large an area. Its use could soon be extended to certain chronic migraines.
Medical cannabis is also used to treat intractable pain in various countries.
Transcutaneous electrical nerve stimulation (TENS) is a technique in which electrodes stuck to the skin can relieve the pain in question.
Electrical stimulation of the spinal cord has also been used for many years, particularly in patients suffering from chronic low back pain. The technique involves implanting electrodes along the dura mater (the membrane surrounding the spinal cord), which are then connected to a stimulator, itself implanted under the patient's skin in the abdomen. The system is controlled by an external remote control that allows the patient to trigger stimulations when the pain increases. These stimulations blur the painful message and reduce its intensity.
Non-pharmacological treatments
Transcranial cerebral neuromodulation is currently being developed. It aims to interact with the pain control mechanism through magnetic or electrical stimulation. These approaches are being developed for severe pain that is resistant to other treatments. The magnetic approach has the advantage of not requiring electrodes to be implanted in the brain. This technique gives good results in fibromyalgia and also in the treatment of neuropathic pain, with no side effects. Studies have shown that the effectiveness of this approach can last for at least 6 months in some patients.
Acupuncture, relaxation therapy, sophrology, magnetotherapy and hypnosis have all played an important role in pain relief centres, and in some patients they can help reduce the need for medication by significantly reducing pain perception.